Understanding the characteristics of geologic formations and fluids located therein is important for effective hydrocarbon exploration and production. Formation evaluation relies on accurate petrophysical interpretation derived from a diverse set of logging technologies. One such technology, nuclear magnetic resonance (NMR), can be used to estimate formation characteristics such as mineralogy-independent porosity and permeability of rocks, to perform fluid typing and determine fluid volumes, and to estimate fluid characteristics such as viscosity.
Processing results from NMR operations have indicated that porosity overcall is a significant problem in formations with short relaxation time T2 (e.g., on the order of milliseconds (ms)) if T1 is greater than T2. For example, NMR measurements in formations having shale intervals have exhibited significant porosity overcall (in the order of 20 percent) in some shale intervals, even though porosity in other regions was found to be correct.